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Muscular System How do muscles work? • Grip a clothespin with the thumb and index finger of your writing hand. Squeeze the clothespin open and shut as fast as you can for 20 seconds. Count how many times you can squeeze the clothespin before your muscles tire. • Rest for one minute. Then repeat. • What happened? What were the differences? Why do you think this is? • Two kinds of muscles: – Voluntary – those under your own control • Examples: – Involuntary – not under your own control, function on their own • Examples: Three types of muscles • Skeletal – Function: movement due to muscles attached to bones with a tendon; muscles work in pairs, one muscle contracts or shortens and the other relaxes – Voluntary – Striated – Examples: Biceps and Triceps Three types of muscles (cont.) • Smooth – Function: Lines the insides of organs to automatically control their movements – Involuntary – Un-striated – Examples: Intestines, and Stomach Three types of muscles (cont.) • Cardiac – Function: lines the heart; “pacemaker” cells control contractions; cardiac muscle never gets tired – Involuntary – Striated – Examples: Heart